1. Field of the Invention
The present invention relates to antennas, more particularly to a dual-frequency antenna for mobile phones.
2. Description of the Related Art
At the beginning of 1980's, European countries jointly established a Group Special Mobile (GSM) research team to conduct research and development for setting up a GSM system standard in order to solve the incompatibility of mobile phone systems in Europe, and the issue of mobile phone users unable to roam around Europe. The GSM research team set up two frequencies within the 900 MHz range for being used by digital mobile phones. Since GSM systems adopt a “digital technology”, therefore, it does not only improve the system capacity, but also effectively solves the problems of not having sufficient capacity for analog systems, communication confidentiality, and data communication capability. The GSM system further solves the issue of incompatibility of mobile phones in different countries. Therefore, the range of GSM applications has grown rapidly since 1992 when it officially started providing services. The GSM system has become one of the most fast-growing types of digital mobile phones, which can be illustrated by the number of existing mobile phone owners. Almost everyone has a mobile phone in these days.
Further, the General Packet Radio Service (GPRS) communication protocol was established to meet the user's requirements for the wireless communications. In fact, the GPRS uses the current GSM communication network architecture to provide a high-speed packet data radio service without replacing all of the current GSM systems, so that users can connect to a wireless network to transmit and receive text and graphic data. The GPRS has a transmission speed of up to 115K per second, and can maintain an online connection nearly all the time. Therefore, more and more people select the mobile phone having the functions of the GPRS system in recent years to transmit and receive data via wireless networking.
In general, a traditional antenna used in GSM mobile phones is limited to a single-frequency antenna. However, the single frequency antenna no longer meets the requirements of the fast development of the diversified communication system. Therefore, the single-frequency antenna is gradually substituted by the dual-frequency antenna that can support dual-frequency systems. The dual-frequency antenna at early stage adopts the exposed design. However, since this kind of antenna is exposed outside the casing of the mobile phone, the mobile phone cannot have a compact design, not only causing inconvenience to the users, but also creating bottlenecks and obstacles on the styling design. Therefore, the dual-frequency antenna used in a dual-frequency mobile phone 10 gradually changes to the built-in dual-frequency antenna 12 as shown in FIG. 1. Although the antenna of such dual-frequency mobile phone 10 is hidden, which can give a better look for the overall design, the dual-frequency antenna 12 is added to the printed circuit board 11 of the mobile phone 10, not only making the assembly inconvenient, but also having the disadvantages of occupying extra space and costing more. Furthermore, the size of mobile phones 10 cannot meet the requirements and trend of a compact design.